Method for the removal of Nox and N2O from the tail gas in nitric acid production

ABSTRACT

A process is described for reducing the NO x  concentration and N 2 O concentration from the residual gas from nitric acid production. The process encompasses the passing of the residual gas leaving the absorption column, prior to entry into the residual gas turbine, through a combination of two stages. The first stage here reduces the NO x  content and the second stage the N 2 O content of the gas, the NO x /N 2 O ratio prior to entry of the gas into the second stage being in the range from 0.001 to 0.5, and this gas being brought into contact in the second stage with a catalyst which is substantially composed of one or more iron-loaded zeolites.

The present invention relates to a process for eliminating NOx and N₂Ofrom the tail gas from nitric acid production.

Industrial production of nitric acid HNO₃ by catalytic combustion ofammonia produces a waste gas loaded with nitrogen monoxide NO, nitrogendioxide NO₂ (together termed NOx), and also nitrous oxide N₂O. While NOand NO₂ have long been recognized as compounds having relevance toenvironment toxicity issues (acid rain, smog formation), worldwidelimits having been set for maximum permissible emissions of thesematerials, the focus of environmental protection has in recent yearsincreasingly also been directed toward nitrous oxide, since it makes anot inconsiderable contribution to the decomposition of stratosphericozone and to the green-house effect.

After the adipic acid industry has reduced emissions of nitrous oxide,nitric acid production is the largest source of industrial emissions ofnitrous oxide. For reasons of environmental protection, therefore, thereis an urgent requirement for technical solutions for reducing nitrousoxide emissions as well as NO_(x) emissions during nitric acidproduction.

There are numerous versions of processes for eliminating NOx from thewaste gas from nitric acid production (termed here the DeNOx stage),such as chemical scrubbing, adsorption process, or catalytic reductionprocesses. Ullman's Encyclopedia of Industrial Chemistry, Vol. A 17, VCHWeinheim (1991) (D1) gives an overview. Emphasis should be given here toselective catalytic reduction (SCR) of NOx by means of ammonia to giveN₂ and H₂O. Depending on the catalyst, this reduction can proceed attemperatures of from about 150° C. to about 450° C., and permits morethan 90% NO_(x) decomposition. This is the version of NOx reductionmostly utilized during nitric acid production, but, like the otherversions, does not lead to any reduction in N₂O content.

For this purpose the current prior art requires a separate, secondcatalyst stage, advantageously combined with the DeNOx stage.

An example of the process based on this approach is described in U.S.Pat. No. 5,200,162, which claims the decomposition of N₂O in a waste gaswhich also comprises NOx, downstream of a DeNOx stage. Here, at leastone substream of the waste gas which leaves the N₂O decomposition stageis cooled and returned thereto in order to avoid overheating of thisstage due to the exothermic nature of the N₂O decomposition process. Theinvention relates to waste gases whose N₂O content is up to 35% byvolume, e.g. to waste gases from adipic acid production.

A process put forward by Shell describes the integrated elimination ofNOx and N₂O in the residual gas from nitric acid production (Clark, D.M.; Maaskant, O. L.; Crocker, M., The Shell DeNOx System: A novel andcost effective NOx removal technology as applied in nitric acidmanufacture and associated processes, presented at Nitrogen '97, inGeneva, 9-11 Feb. 1997, (D2)).

The Shell reactor system is based on what is called a lateral flowreactor principle, where even relatively low temperatures (from 120° C.)are possible for the operation of the DeNOx stage. An amorphous metaloxide catalyst is used for removing N₂O.

When appropriate catalysts are arranged in the residual gas leaving theabsorption column with a temperature of from 20 to 30° C., the latitudefor possible operating temperatures is prescribed by the operatingtemperature of the residual gas turbine.

Specifically, for reasons associated with the technical and economicrunning of the entire process, the residual gas turbine should mostadvantageously be operated with entry temperatures <550° C. and withmaximum ΔT and Δp.

This is particularly important for eliminating N₂O, since according tothe current prior art this requires markedly higher temperatures thanthose needed during catalytic reduction of NOx. The cost-effectivenessof this option is therefore linked to adequate catalyst activity.

Kapteijn F.; Rodriguez-Mirarsol, J.; Moulijn, J. A., App. Cat. B:Environmental 9 (1996) 25-64, (D3) gives an overview of the numerouscatalysts which have been demonstrated to be suitable in principle fordecomposing and reducing nitrous oxide.

Metal-exchanged zeolite catalysts (U.S. Pat. No. 5,171,533), inter alia,appear particularly suitable for decomposing N₂O.

The zeolites used here are prepared by ion exchange in an aqueoussolution comprising metal salts. The metals used for the ion exchangeare from the group: copper, cobalt, rhodium, iridium, ruthenium, andpalladium. The copper zeolites are highly sensitive to water vapor andrapidly lose their activity under those conditions (M.; Sandoval, V. H.;Schwieger, W.; Tissler, A.; Turek, T.; Chemie Ingenieur Technik 70(1998) 878-882, (D5)), while the other metals are listed here arerelatively expensive.

Using iron-doped zeolite of Fe-ZSM5 type under appropriate conditions,as described in Table 1 in U.S. Pat. No. 5,171,533, in the absence ofNOx, H₂O, and O₂ at 450° C., only 20% N₂O decomposition was achieved.

The activity of Fe-ZSM-5 for decomposing N₂O is, however, markedlyincreased in the presence of appropriate amounts of NO, this beingattributed to a reaction forming NO₂ as in NO+N₂O N→N₂+NO₂, catalyzed byFe-ZSM-5 (Kapteijn F.; Marban, G.; Rodriguez-Mirasol, J.; Moulijn, J.A.; Journal of Catalysis 167 (1997) 256-265, (D6); Kapteijn F.; Mul, G.;Marban, G.; Rodrigeuez-Mirasol, J.; Moulijn, J. A., Studies in SurfaceScience and Catalysis 101 (1996) 641-650, (D7)).

In the absence of NO_(x), higher activity was found for Cu orCo-exchanged zeolites than for the corresponding Fe zeolites.

In the descriptions set out in the prior art (D6, D7) of N₂Odecomposition in the presence of an Fe-ZSM-5-catalyst at 400° C., use isusually made of equimolar amounts of NO and N₂O. In D6 and D7, theeffect of NOx and N₂O decomposition falls constantly as NO/N₂O ratiosinks, and therefore N₂O decomposition becomes unsatisfactory when theNO/N₂O ratio is below 0.5.

The best results are found when the molar ratio NO/N₂O is 1 or greaterthan 1.

According to the authors, when this catalyst is used for N₂O reductionin the waste gas from nitric acid production, the NO₂ formed could bereturned to the process for obtaining HNO₃. Depending on the version ofthe process, the NOx concentration and N₂O concentration in the wastegas are about 1 000 ppm.

WO 99/34901 relates to iron-containing zeolites based on ferrierite forreducing N₂O-containing gases. The catalysts used here comprise from 80to 90% of ferrierite, and also binders. The water content of the gasesto be reduced is in the range from 0.5 to 5%. When various zeolite typesare compared, the best results for decomposition of N₂O at temperaturesof from 375 to 400° C. were obtained using zeolites of FER (ferrierite)type (97% N₂O decomposition at 375° C. and NO/N₂O=1). Substantially lessdecomposition was found when using zeolites of pentasil (MFI) type ofmordenite (MOR) type. Indeed, the maximum N₂O decomposition achievableunder the above conditions when iron-containing MFI zeolites were usedwas only 62%.

In the light of the known prior art, it is therefore an object toprovide an economic process, in particular for HNO₃ production, whichpermits not only high levels of NO_(x) decomposition but alsosatisfactory N₂O decomposition.

In particular, good results for N₂O decomposition are to be obtainedeven when the NOx/N₂O ratio is substoichiometric, in particular at theratios which result after NOx content reduction, i.e. at a ratio <0.5,preferably <0.1.

The present invention achieves this object and provides a process forreducing the NO_(x) concentration and N₂O concentration from the tailgas from nitric acid production, where the tail gas leaving theabsorption column is passed, prior to entry into the tail gas turbine,through a combination of two stages, the first stage (DeNOx stage)reducing the NOx content and the second stage (DeN₂O stage) reducing theN₂O content of the gas, and where the NOx/N₂) ratio prior to entry ofthe gas into the second stage [lacuna] in the range from 0.001 to 0.5,preferably in the range from 0.001 to 0.2, in particular in the rangefrom 0.01 to 0.1, and in the second stage this gas is brought intocontact with a catalyst which is substantially composed of one or moreiron-loaded zeolites.

Catalysts used according to the invention are composed substantially ofone or more iron-loaded zeolites, preferably >50% by weight, inparticular >70% by weight. For example, alongside an Fe-ZSM-5 zeolitethere may be another iron-containing zeolite present in the catalystused according to the invention, e.g. an iron-containing zeolite of theMFI type or MOR type. The catalyst used according to the invention maymoreover comprise other additives known to the skilled worker, e.g.binders.

The catalysts used for the DeN₂O stage are preferably based on zeolitesinto which iron has been introduced via solid-phase ion-exchange. Theusual starting materials here are the commercially available ammoniumzeolites (e.g. NH₄-ZSM-5) and the appropriate iron salts (e.g.FeSO₄×7H₂O), these being mixed intensively with one another bymechanical means in a bead mill at room temperature. (Turek et al.;Appl. Catal. 184, (1999) 249-256; EP-A-0 955 080). These citations areexpressly incorporated herein by way of reference. The resultantcatalyst powders are then calcined in a furnace in air at temperaturesin the range from 400 to 600° C. After the calcinations process, the Fezeolites are thoroughly washed in distilled water, and the zeolites arefiltered off and dried. The resultant Fe zeolites are finally treatedwith the suitable binders and mixed, and extruded to give, for example,cylindrical catalysts bodies. Suitable binders are any of the bindersusually used, the most commonly used here being aluminum silicates, e.g.kaolin.

According to the present invention, the zeolites which may be used areiron-loaded zeolites. The iron content here, based on the weight ofzeolite, may be up to 25%, but preferably from 0.1 to 10%. Particularlysuitable zeolites here are of the type MFI, BETA, FER, MOR, and/or MEL.Precise details concerning the build or structure of these zeolites aregiven in the Atlas of Zeolithe Structure Types, Elsevier, 4th revisedEdition 1996, which is expressly incorporated herein by way ofreference. According to the invention, preferred zeolites are of MFI(pentasil) type or MOR (mordenite) type. Zeolite Fe-ZSM-5 type areparticularly preferred.

According to the present invention, DeN₂O catalysts are arranged incombination with an upstream DeNOx stage, between the absorption columnand the tail gas turbine, in such a way that the tail gas leaving theabsorption column is first passed at temperatures <400° C., inparticular <350° C., into a reactor (first stage) in which the NOxcontent of the gas is reduced to <100 ppm (cf. FIG. 2). The operatingpressure for this first stage is preferably from 1 to 15 bar, inparticular from 4 to 12 bar.

The upstream DeNOx stage corresponds to a process usually used in nitricacid plants of the prior art for reducing the amount of NOx emissions.However, the remaining NOx content of the tail gas has to besufficiently high to permit the cocatalytic effects of NO or NO₂ to beactive in the downstream DeN₂O stage.

If the DeN₂O stage is operated without upstream DeNOx, i.e. if theentering stream has approximately equimolar amounts of NO and N₂O,return of the NO₂ formed by NO+N₂O N₂+NO₂ into the HNO₃ process isuneconomic, due to the relatively low NO₂ concentration, <2 000 ppm.

The N₂O content of the gas remains substantially unaltered in the DeNOxstage. After leaving the first stage, therefore, the NO_(x) content ofthe gas is usually from 1 to 200 ppm, preferably from 1 to 100 ppm, inparticular from 1 to 50 ppm, and its N₂O content is from 200 to 2 000ppm, preferably from 500 to 1 500 ppm. The resultant NOx/N₂O ratio afterleaving the DeNOx stage is from 0.001 to 0.5, preferably from 0.001 to0.2, in particular from 0.01 to 0.1. The water content of the gas, bothafter leaving the absorption column and, respectively, the DeNO_(x)stage and after leaving the DeN₂O stage, is usually in the range from0.05 to 1%, preferably in the range from 0.1 to 0.8%, in particular inthe range from 0.1 to 0.5%.

The tail gas conditioned in this way is then passed into the downstreamDeN₂O stage, where decomposition of the N₂O into N₂ and O₂ is broughtabout by utilizing a cocatalytic effect of NOx in the presence of theappropriate zeolite catalyst.

Surprisingly, it was found that in the presence of the iron-containingzeolite catalysts used according to the invention N₂O decomposition isdrastically increased (cf. FIG. 1) even in the presence of small amountsof NOx, i.e. when the molar NOx/N₂O ratio is <0.5. An effect whichbecomes markedly more pronounced as the temperature increases. Accordingto the present invention, therefore, at 450° C., for example, a molarNO_(x)/N₂O ratio of 0.01 is still sufficient to lower the N₂Oconcentration from 72% to 33% in the presence of an Fe-ZSM-5 catalyst.This is all the more astounding since in the prior art the accelerateddecomposition of N₂O is attributed to the abovementioned stoichiometricreaction of N₂O with NO. If the temperature is sufficient it appearsthat if the NO_(x)/N₂O ratio is small NO_(x) adopts the role of ahomogeneous cocatalyst which accelerates N₂O decomposition as inN₂O→N₂+½O₂. If the NO_(x)/N₂O ratio is within the abovementioned limits,maximum decomposition of N₂O is possible in the downstream DeN₂O stage.As soon as the ratio falls away below 0.001, N₂O decomposition alsosinks to values which become unsatisfactory (cf Example 5). The contentof N₂O in the process of the invention after leaving the DeN₂O stage isin the range from 0 to 200 ppm, preferably in the range from 0 to 100ppm, in particular in the range from 0 to 50 ppm.

The operating temperature for the DeN₂O stage here is in particulardetermined by the desired degree of decomposition of N₂O and the amountof NOx present in the tail gas, but also, as is known to the skilledworker and like almost all catalytic waste gas purification processes,highly dependent on the catalyst loading, i.e. on the waste gasthroughput based on the amount of catalyst. The operating temperaturefor the second stage is preferably in the range from 300 to 550° C., inparticular the range from 350 to 500° C., the pressure being in therange from 1 to 15 bar, in particular from 4 to 12 bar. As pressurerises, the cocatalytic action of NOx on N₂O decomposition becomesgreater, and increase of pressure therefore permits a further drop inoperating temperature.

In determining or setting the operating temperature, account also has tobe taken of the content of oxygen and H₂O. This content can vary withincertain limits, depending on the mode of operation and on the version ofthe process used for nitric acid production, and inhibits N₂Oconversion. The O₂ content is in the range from 1 to 5% by volume, inparticular in the range from 1.5 to 4% by volume.

N2O decomposition of >90%, in particular >95%, can therefore be achievedat temperatures in the range from 300 to 550° C., preferably from 350 to500° C., using the iron-containing zeolite catalysts used according tothe invention. As temperature rises it is even possible to achievesatisfactory N₂O decomposition when the NO_(x)/N₂O ratio is 0.01.

By combining a DeNOx stage and a DeN₂O stage, the process of theinvention permits the NOx content and N₂O content of the tail gas to bereduced to minimal values during nitric acid production. The arrangementof the DeNox stage prior to the DeN₂O stage and between absorptioncolumn and tail gas turbine moreover makes the process of the inventionvery economic, due to the continuously rising temperature profile.

Furthermore, the arrangement of both stages prior to the decompressionturbine makes the conduct of the process particularly advantageous,since both stages can be operated at superatmospheric pressure (between4 and 11 bar, depending on the version of the HNO₃ process), resultingin a reduction of the volume of reactor and, respectively, catalysteffectively needed.

Furthermore, since the DeNOx stage operates even at relatively lowtemperatures, sufficient reduction of NO_(x) content during plantstart-up is also ensured when only little process heat is available.

Another advantage of arranging both stages between absorption column andtail gas turbine in a continuously rising temperature profile is thatthe tail gas leaving the inventive combination can be introduced,without prior cooling, and without any other measures for waste gaspurification, directly to the tail gas turbine for ideal reclamation ofcompressive and thermal energy.

EXAMPLES DeNOx Stage

The DeNOx catalyst used as described with NH₃ as reducing agent upstreamof the DeN₂O catalyst was a conventional SCR catalyst based on V₂O₅ ⁻WO₃ ⁻ /TiO₂ (cf., for example, G. Ertle, H. Knözinger J. Weitkamp:Handbook of Heterogeneous Catalysis, Volume 4, pages 1633-1668). Thiswas operated at a temperature of 350° C. Depending on the amount of NH₃introduced, various NO_(x) contents and therefore NO_(x)/N₂O ratios wereset at the outlet from the DeNOx stage.

DeN₂O Stage

An iron-containing MFI catalyst was prepared by solid-phase ionexchange, starting from a commercially available ammonium-form zeolite(ALSI-PENTA, SM27). Detailed information concerning the preparationprocess may be obtained from: M. Rauscher, K. Kesore, R. Mönnig, W.Schwieger, A. Tiβler, T. Turek, Appl. Catal. 184 (1999) 249-256.

The catalyst powders were calcined in air for 6 h at 823 K, washed anddried overnight at 383 K. Extrusion to give cylindrical catalyst bodies(2×2 mm) followed after addition of appropriate binders.

The experiments were carried out in a flux apparatus operated at steadystate with on-line analysis, the space velocity in each case being 10000 h⁻¹.

The composition of the feed was

-   -   1 000 ppm NO_(x,)    -   1 000 ppm N₂O    -   0.5% vol H₂O    -   2.5% vol O₂    -   remainder N₂

The following residual concentrations of NO_(x) and N₂O were obtained byvarying the amount of NH₃ added:

Resultant NO_(x) Resultant Resultant N₂O concentration NO_(x)/N₂Oconcentration Ex- (after DeNOx ratio (after (after DeN₂O am- Amount ofstage at DeNOx stage at ple NH₃ added 350° C.) stage) 475° C.) 1 500 ppm500 ppm  0.5 40 ppm 2 800 ppm 200 ppm  0.2 54 ppm 3 950 ppm 50 ppm 0.0581 ppm 4 990 ppm 10 ppm 0.01 99 ppm 5 1000 ppm  <1 ppm <0.001 462 ppm 

As can be seen from the examples given, a high level of N₂Odecomposition is possible up to an NO_(x)/N₂O ratio of 0.001, inparticular 0.01. If the ratio sinks below this limit, the decompositionlevel becomes inadequate, since there is no longer sufficientcocatalytic function of NO_(x).

1. A process for reducing the NO_(x) concentration and N₂O concentrationfrom the tail gas from nitric acid production using an absorption columnand a tail gas turbine, where the tail gas leaving the absorption columnis passed, prior to entry into the tail gas turbine, through acombination of two stages, the first stage reducing the NO_(x) contentby catalytic reduction, and the second stage reducing the N₂O content ofthe gas by decomposition into nitrogen and oxygen, and where the molarNO_(x)/N₂O ratio prior to entry of the gas into the second stage is inthe range from 0.01 to 0.5, and in the second stage this gas is broughtinto contact with a catalyst which comprises one or more iron-loadedzeolites, the operating pressure in the second stage being from 4 to 12bar, with the proviso that said one or more iron-loaded zeolites doesnot comprise FER type iron-loaded zeolite.
 2. The process as claimed inclaim 1, characterized in that the iron-loaded zeolite(s) present in thecatalyst are of MFI, BEA, MOR and/or MEL type.
 3. The process as claimedin claim 2, characterized in that the iron-loaded zeolite (s) are of MFItype.
 4. The process as claimed in claim 3, characterized in that thezeolite is an Fe-ZSM-5.
 5. The process as claimed in claim 1,characterized in that the temperature of the first stage is <400° C. 6.The process as claimed in claim 1, characterized in that the temperatureof the second stage is in the range of 300 to 550° C.
 7. The process asclaimed in claim 1, characterized in that both stages are operated at apressure in the range of from 4 to 12 bar.
 8. The process as claimed inclaim 1, characterized in that the first stage is operated using the SCRprocess.
 9. The process as claimed in claim 1, characterized in that,after leaving the absorption column and prior to entry into the first orsecond stage, use is made of the tail gas whose water content is in therange from 0.05 to 1% by volume.
 10. The process as claimed in claim 1,characterized in that, prior to entry into the second stage, NO_(x)content of the gas is in the range from 1 to 200 ppm and the N₂O contentof the gas is in the range from 200 to 2000 ppm.
 11. The process asclaimed in claim 5, characterized in that the temperature of the firststage is <350° C.
 12. The process as claimed in claim 6, wherein thetemperature of the second stage is in the range of 350 to 500° C. 13.The process as claimed in claim 9, wherein the water content of the tailgas is in the range from 0.1 to 0.8% by volume.
 14. The process of claim1, wherein the catalyst comprises greater than 50% by weight of saidiron-loaded zeolites.
 15. The process of claim 1, wherein the catalystcomprises greater than 70% by weight of said iron-loaded zeolites. 16.The process of claim 14, wherein the iron-loaded zeolite(s) are of theMFI type.
 17. The process of claim 15, wherein the iron-loadedzeolite(s) are of the MFI type.
 18. The process of claim 16, wherein theiron-loaded zeolite is an Fe-ZSM-5.
 19. The process of claim 17, whereinthe iron-loaded zeolite is an Fe-ZSM-5.
 20. The process as claimed inclaim 1, wherein the molar NO_(x)/NO₂ ratio is less than 0.1.
 21. Theprocess as claimed in claim 1, wherein the molar NO_(x)/NO₂ ratio isfrom 0.01 to 0.1.
 22. The process as claimed in claim 1, wherein thefirst stage catalytic reduction reduces the NO content to less than 100ppm.